Gate Obsidian Toolbox behind TOOLBOX_VISIBLE flag

Toolbox is still being refined; hide it from clients on prod while keeping it
usable on staging. Entry points (header link, dashboard card) render only when
TOOLBOX_VISIBLE=true; a tools/layout redirects /tools/* to /dashboard otherwise.
Routes and ToolUsage table are untouched (no destructive migration).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/(student)/tools/layout.tsx

@@ -0,0 +1,8 @@
+import { redirect } from "next/navigation";
+
+// Toolbox ещё в доработке: на проде скрыт (флаг не задан) — прямой заход на
+// /tools/* уводит на дашборд. На staging выставлен TOOLBOX_VISIBLE=true.
+export default function ToolsLayout({ children }: { children: React.ReactNode }) {
+  if (process.env.TOOLBOX_VISIBLE !== "true") redirect("/dashboard");
+  return <>{children}</>;
+}